rocket stove and butt warmer

Yesterday i made my first rocket stove. I used scrap gypsum drywall board. It worked pretty good but wasn't scale. So last night i made another out of broken rocks from my flower bed and some clay sewer pipe. it works very well. yet i still need more practice. I am going to make a mold an cast one. Good thing I Plaster for a living.

charles c. johnson

Joined: Dec 02, 2009
Posts: 369

posted Dec 04, 2009 05:38:09

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The reason i asked about alternative fuels is I'm cheap lol. Also We go out of town in the winter and I'm worried about my dogs and water pipes freezing. I'm refinishing a 1920's town house. I want the rocket stove in the center of the basement. I hope that i can make my thermal mass long wide and heavy. my goal is 6 tons of earth. or 12000lb. Thankfully a ground hog has all ready put 3 tons of clay in the basement. I know you said paul's vent was like a dryer . But once your thermal mass hits temp does the out gas heat increase>?

carbonout wrote: The reason i asked about alternative fuels is I'm cheap lol. Also We go out of town in the winter and I'm worried about my dogs and water pipes freezing. I'm refinishing a 1920's town house. I want the rocket stove in the center of the basement. I hope that i can make my thermal mass long wide and heavy. my goal is 6 tons of earth. or 12000lb. Thankfully a ground hog has all ready put 3 tons of clay in the basement. I know you said paul's vent was like a dryer . But once your thermal mass hits temp does the out gas heat increase>?

Probably it does, logically speaking. But the temperature of the thermal mass never really gets above about 130 F at the hottest on the outside. Seems like the gas stays in the "damp and warm" temperature range even when the stove is quite warm - I remember putting my face in one at Cob Cottage and it was a pleasant experience.

I'd go measure ours for you, but it's vented with a conventional steel chimney and our roof is too steep for my comfort.

Ernie thinks that maybe 300-400 degrees F would be the hottest you'd expect, on a stove with typical mass and size proportions. This is still in the range of temperatures I'd expect inside a clothes dryer (Medium to High settings), but might be a little hotter than the typical dryer exhaust.

I tend to set up my exhaust thru-wall like a woodstove, if possible. Insulated pipe, or a triple-wall section. I just feel better doing it that way, just in case. Maybe someone will tear out my stove and replace it with a woodstove, or something, and I don't want them to get a nasty surprise.

But I know a lot of people have set them up like a big dryer vent and I haven't heard of that causing any problems from temperature. The most common problem is if you don't screen it and animals crawl in between burn seasons.

I tend to set up my exhaust thru-wall like a woodstove, if possible. Insulated pipe, or a triple-wall section. I just feel better doing it that way, just in case.

In case of what?

I would think that at the end of the burn, stuff would stop moving through the pipe. And then you would have a lot of steam and CO2 sitting in that vertical pipe. And then, when things cool, the flow will reverse into the room. I would think that the lower the exit vent the better.

IMO its just the precaution if the stove for some reason is burning to hot. (ok i tend to do test stuff on our stove like feed it veggy oil and get it up around 900 degree on the surface. Erica is just protecting us from me )

if you dont do the crazy stuff i do it should not be a problem as long as your mass can absorb the heat. and you exhaust is around 100 degrees. you just dont want your exhaust at ignition temps. so shoot for 90 degrees exhaust temp and you will be fine with a vertical stack and about 70 degrees for a horizontal exhaust.

charles johnson "carbonout" wrote: do you drip the oil in ? do you use a wick ?

it was a test for an oil drip like a diesel stove. it works but you have to get the drip right or it smokes out the area. working on the problem as i go.

Joined: Jan 18, 2010
Posts: 5

posted Jan 18, 2010 19:39:17

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I happened on this site while looking for HASU plans that seem to have disappeared. The principle of thermal mass storage is similar except 16 tons of sand and a piping network made a heat exchange. I really like this simplified version.

Joined: Jan 28, 2010
Posts: 1

posted Jan 28, 2010 08:00:47

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Paul,

Question – how do you clear the ash and waste after burning. The only opening I see is the fuel feed at the top.

befor you start the days fire just use your hand. we are trying a thing looks like a cat box cleaner but your hand is still the best tool. not many coals just a little ash.

Joined: Jan 18, 2010
Posts: 5

posted Feb 10, 2010 19:08:21

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I am interested in building a rocket stove to heat a green house. I am proposing to use the rocket as a single heat source to feed two discharge through mass. Imagine a large block shaped letter S. top & bottom run would be 21 feet long, the connector's would be 8 Ft & the center frun would be 18 feet with the stove connecting to the center entering a tee so discharge could be routed either way by capping the end not desiring to use. The total run of mass would be 75ft but only 37 1/2 per side. The bench to absorb heat will be 18x18 with raised beds 8" deep x 24" wide on both sides. I am looking for reasons this design will not work

pete i have no idea how its going to work. the longest run so far is 50 feet in an 8 inch system; i think you might be pushing it alot for the size if install you want.

I would be thinking a 10 inch system for that span and amount of Thermal mass.

Joined: Jan 18, 2010
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posted Feb 21, 2010 00:20:58

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The way I am thinking my longest run would be 40ft max. I am proposing using one stove for two flues going different directions and ending in different areas. I would need to be able to cap or plug the flue not in use to direct it to the other flue. I am now thinking I would need a way to also close one flue near to the stove but as yet not sure how or decided.

Ernie Wisner
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posted Feb 21, 2010 18:27:11

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Ahh now it comes clear. ok that would be no problem. the way to do the selection to the sides is simply to put in a valve. trying to fiddle the manifold itself is adding complexity where yo dont need it. but running the exhaust through a gated T is totally doable.

Ernie wrote:trying to fiddle the manifold itself is adding complexity where yo dont need it. but running the exhaust through a gated T is totally doable.

Capping the exhaust would not be perfect, but might it be enough? Some heat would be drawn into the branch you don't intend, but probably not a majority of it, right?

"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.

Ernie Wisner
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posted Feb 23, 2010 11:31:43

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actually no need for the cap if you get a valved T. they make them for ventilation systems and if you replace the foam gasket with a leather one the seal will be pretty good. as to the heat the exhaust gasses are going to use the path of least resistance so you would be fine. my concern would be gas build up in the capped end CO2 is heavy so its going to displace the lighter gasses in the capped off leg. this might be a good thing if you are exhausting out of the space cause you can use the flow of the heavy gas to prime the pump so to speak. but i would watch it closely and make sure it was not a problem. the valve would be better cause all the controls would be at the stove making it more likely you would use the system and the gases could not build up in the closed off leg.

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Posts: 5

posted Feb 23, 2010 15:26:23

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My intention is to have a slight grade down and out. I was thinking that the cap on the end with a gated Tee would allow the mass longer to dissipate heat with out cooling it from the inside as when open to atmosphere

Ernie Wisner
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posted Feb 23, 2010 20:43:28

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that sounds like a plan. i would still check the off leg just to be sure. but i am a bit funny about unintended effects.

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posted Feb 25, 2010 03:08:28

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hi everybody.one thing i want to ask about the whole process is what is the distance from the exit of the combustion chamber and the top of the barrel and how important is that

Do forgive me if I'm mentioning something that has already been mentioned (I did not take-- do not have-- the time to read all the posts), but surely it's important that someone mention that the efficiency of any stove will depend on the system, not just the stove.

That is, any stove that is inside a shelter (greenhouse, house, etc.) but which get its air-to-burn from inside that structure, will create a partial vacuum inside that shelter. In turn, outside air will have to be pulled into the structure, cooling it, and the stove will have to use some of its output to redress the balance.

Thus one good way to improve the efficiency of the stove system is to make sure that what feeds the stove oxygen is air taken directly from the outside.

Now, as it turns out, the best way to get what would otherwise be cold air into the combustion chamber is to draw it down (and around) the outside of the chimney. That is, one has the hot exhaust pipe entirely surrounded by an air-filled pipe which draws air from above the roof (in a manner protected from the rain, etc.) and feeds it down into the firebox.

What happens in the situation described, if you think about the temperature of the exhaust stream, is that down near the bottom as it pushes up to exit, it is hottest. It heats the wall of its pipe, and that heat is transferred to the incoming air-to-be-burned, so it keeps putting the heat back into the firebox. More of the heat will then escape is into the shelter, and less will escape out into the air.

This sort of heat transfer is most efficient where the temperatures of the two gas streams (separated by the exhaust pipe wall) are as great as possible. That is, where the exhaust is as hot as it can be and the incoming air is as cold as it can be, that much more heat per square unit surface or unit time will be pushed across the barrier of the pipe wall and surface. (The Complete Biogas Handbook, do forgive the plug, has a complete explanation of all this, but for the moment...)

And if you think about it, and try to visualize it (I will not attempt to make or re-make Paul's drawing), you will see that up at the top of the doubled pipe, the exhaust is as cool as it will get while we still have it in our grasp. At the same time, the incoming air is also as cool as it will be, meaning that the temperature difference between the two is as much as we can reasonably expect. Likewise, in the middle of the pipe, the incoming air has heated somewhat, but the exhaust is also hotter. And down at the bottom, whereas the incoming air has been heated still more, the exhaust is likewise as hot as it will ever be. So at all points along the pipe, the temperature difference is as great as it can be between the two gases. (This is called countercurrent flow, by the way, because the two gases are traveling in different directions. Whales use the same strategy with blood flow from their flippers into their body to retain heat. That is, the veins and arteries are tangled around one another, and keep the heat within the better-insulated body cavity. You can look it up!)

Where the two gases are traveling in the same direction, then they can reach equilibrium at some point along their journey, and the exhaust gas will then no longer heat the incoming air-to-burn, because they are at the same temperature.

Again think of the whole system, and you will see that the ultimate effect, once more, is that heat is retained in the shelter, and no cold air is pulled in to the shelter to feed the fire. The efficiency of the whole system then-- which will translate into how much wood you have to chop, carry and feed the stove-- is getting better and better. The stove, then will use less wood to produce the same heat, or will produce more heat with the same wood.

That particular issue has been discussed many, many times. The key difference is to consider the many types of heat and the value of a thermal mass. Take a look at this thread.

David House

Joined: Feb 19, 2010
Posts: 34
Location: Oregon, USA

posted Mar 01, 2010 14:10:02

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Paul,

paul wheaton wrote: David,

That particular issue has been discussed many, many times. The key difference is to consider the many types of heat and the value of a thermal mass. Take a look at [url=http://www.permies.com/permaculture-forums/2831_0/alternative-energy/wofati-plus-a-rocket-mass-heater]this thread .

I appreciate the thread link. Thanks.

From reading in the referenced thread, however, it seems there is a confusion between having an airtight house and getting air from outside to burn. These are two separate subjects. Where one has an airtight house and a stove (or even worse, a fireplace) that burns inside air, the most dangerous situation is created, because the chimney of the stove/fireplace may become an air inlet if some other appliance or heater is exhausting air to the outside, creating a partial vacuum. When that happens during the time a fire in the stove/fireplace is dying, then while it is not creating a great deal of heat to force air up and out, it is (generally) creating a lot of carbon monoxide, which can be drawn into the house by the partial vacuum. Granted, such problems are worse for an open fireplace than for a well-designed stove, but they exist for either.

Regardless, then, it is never a good idea, as someone in the thread said (quoting you, I think) to live in a zip lock bag.

What makes more sense according to the physics is to have a house where the amount of incoming air can be controlled, and better, brought into the proper temperature range before it enters the house, as possible. One way to do this is to button up the house itself as well as it can be done, and to provide an air inlet via a pipe that brings outside air in from an earth-warmed heat exchanger, which will offer cooler air in the summer and warmer air in the winter. ( http://geoexchange.sustainablesources.com/ )

Further, as mentioned, having a counter-current flow for incoming air provides the highest efficiency. I mention this again because while, as you pointed out, the issue of outside air-to-burn has been discussed, Google does not provide me with any evidence that counter-current flow has been discussed on permies.com. You, however, would know far better than I would.

d.

d.

Joined: Feb 22, 2010
Posts: 25

posted Mar 01, 2010 17:28:32

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there is a more complicated and more costly alternative to drawing combustion air through a second flue channel - and that is to draw it through a buried earth-tube - this tube will preheat the outside air to some degree due to the heat of the soil itself at depth - the benefit of doing things this way is that you will be cooling a large mass of earth while you operate the stove - then, come summertime, air drawn or forced through this same tube will be much cooler than ambient outside air - a substitute for or adjunct to air-conditioning -

Ernie Wisner
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posted Mar 04, 2010 10:58:45

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i dont get the rather better thing. better for what? combustion? the stove itself? you? everyone?

so far i have done the experimenting and not seen anything better then letting the stove have the air it needs. you seem to think it easy to put a pipe with the same cross sectional area as the system through a wall. its not nor is it easy to force feed these stoves with a smaller intake. even a kiva trench is pretty darn big to admit the proper amount of air for the tiny cooking fire.

what are you trying to accomplish by giving the stove its own air intake?

Joined: Feb 22, 2010
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posted Mar 04, 2010 12:29:10

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Ernie wrote: i dont get the rather better thing. better for what? combustion? the stove itself? you? everyone?

so far i have done the experimenting and not seen anything better then letting the stove have the air it needs. you seem to think it easy to put a pipe with the same cross sectional area as the system through a wall. its not nor is it easy to force feed these stoves with a smaller intake. even a kiva trench is pretty darn big to admit the proper amount of air for the tiny cooking fire.

what are you trying to accomplish by giving the stove its own air intake?

i assume your question is not addressed to me - therefore, i shall not answer it -

I think some folks will wanna live in a ziplock bag and add in controls to regulate air flow.

Other folks will wanna skip the expense and effort of the ziplock, and the expense and effort of the airflow control. The might also find other interesting advantages to that (as discussed, I think in that other thread).

Therefore, I choose to not make a claim that one should be embraced as "best". Which could, I worry, lead to the other as being legally banned.

For myself, I used to be of the ziplock bag mentality, and now I am against the ziplock bag mentality.

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posted Mar 08, 2010 16:33:56

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I was privy to Paul's presentation about these amazing stoves at the permaculture design course we attended in February....and the book just arrived in the mail! We plan to build one of these in our cabin this summer. I think it will work well in our space. Thanks for sharing this information, Paul. I'm looking forward to when we can show our neighbors the better way to burn wood for heat.

I am reading a downloaded version of the RMH book and have a couple of questions.

1) Wherever the flue ducts meet the cob there is a statement to the effect that you must be sure to seal the joint tightly so as to prevent CO migration into the home. With cob becoming rigid after heating, becoming ceramic, and the steel of the flue pipes expanding and contracting during heating cycles how does one ensure long term seals? If it is just making sure the cob seals tightly when cold and allowing the greater expansion of the metal during heating to maintain the seal would appreciate that clarification.

2) What materials/methods have been used to raise the RMH the recommended 4" above a wooden floor(diagram on pg 62 of the pdf download), metal studs perhaps with a high temperature gasket as a thermal break between the stud and the floor?

3) Living in NE Ontario I plan a bale-cob home, 4-8" (I could go thicker) of cob inner wall for thermal mass and an outer straw bale insulation. Can a RMH be built against such a wall due to the straw being flammable, would I need separation from such a wall or can the wall be used as part of the thermal battery?

It can be done!

Ernie Wisner
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posted Mar 22, 2010 13:56:32

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"1) Wherever the flue ducts meet the cob there is a statement to the effect that you must be sure to seal the joint tightly so as to prevent CO migration into the home. With cob becoming rigid after heating, becoming ceramic, and the steel of the flue pipes expanding and contracting during heating cycles how does one ensure long term seals? If it is just making sure the cob seals tightly when cold and allowing the greater expansion of the metal during heating to maintain the seal would appreciate that clarification."

there are two methodologies that conflict in that chapter. my own and Ianto's. while the cob seal around the pipe and the pipe seal will work and is very good at not allowing CO2 into the room and is much cheaper (Ianto). I prefer to tape the joints to ensure the seal is totally CO2/air tight. either method will work; I just like the insurance of an aluminum taped/sealed duct.

"2) What materials/methods have been used to raise the RMH the recommended 4" above a wooden floor(diagram on pg 62 of the pdf download), metal studs perhaps with a high temperature gasket as a thermal break between the stud and the floor?"

stone or a few inches of perlite clay insulation you can put the hot part on legs if you like; building on a sheet of hardy backer or something that is supported on a stone foundation. i find the insulation works better for me. I dont advise putting a rocket stove on a wood floor without an engineer to check out how much you can load the floor can carry. We are talking about a 6 ton object it is not a little load for a floor.

"3) Living in NE Ontario I plan a bale-cob home, 4-8" (I could go thicker) of cob inner wall for thermal mass and an outer straw bale insulation. Can a RMH be built against such a wall due to the straw being flammable, would I need separation from such a wall or can the wall be used as part of the thermal battery?"

yes all you need be careful of is the insulation on the out side of the burn tunnel and air gap between the barrel and the wall. you will be putting 4 inches of insulation around the burn tunnel and have a barrel the radiates heat its a big object you will not want to put the radiant heat source in the wall. but the rest of the ducting does not need to be isolated from the wall. the ducting from the manifold on does not get hot enough to burn anything.

Erica and i will be doing a history of fire/ rocket stove workshop in your area this spring. you might want to come by.

its about halfway down and slightly west of the middle of ontario province IIRC.

Not so bad, between Barrie and Owen Sound. Only 6.5 hrs drive away but a LOT closer than the PNW! That's considered Southern Ontario, hack spat ptoey, by the way but at least it's rural S. Ontario and the Grey-Bruce area which isn't too bad . Not too far (~1.5 hrs) from my in-laws in Cambridge.

Dates are confusing, last registration after the course?, if it's during the week as it seems I won't be able to get away from my teaching duties, used up too much time already this year. Anything going on in the summer, July and Aug?? Yes more than 6hrs for you!!